Choose the statement that is true concerning hemoglobin.
A When hemoglobin is not bound to oxygen, it appears blue.
B It is found in the plasma portion of blood.
C It can bind a maximum of three oxygen molecules.
D It is composed of four protein chains and four heme groups.
The correct answer and explanation is :
The correct answer is D: It is composed of four protein chains and four heme groups.
Explanation:
Hemoglobin is a complex protein found in red blood cells (RBCs) that is responsible for transporting oxygen from the lungs to the tissues and carbon dioxide from the tissues back to the lungs. It plays a critical role in maintaining the body’s oxygen supply.
Hemoglobin consists of four protein subunits, each of which is bound to a heme group. Each heme group contains an iron atom that can bind to one molecule of oxygen. Therefore, a single hemoglobin molecule can carry up to four molecules of oxygen—one per heme group. This is in contrast to the incorrect statement that hemoglobin can bind only three oxygen molecules (option C), as each hemoglobin molecule can indeed carry four oxygen molecules.
Hemoglobin’s structure is made up of two alpha and two beta globin chains (in adult hemoglobin, HbA), and each globin chain is associated with one heme group. The heme groups allow hemoglobin to bind oxygen in the lungs, where oxygen concentration is high, and release it in tissues, where oxygen concentration is low. This ability to bind and release oxygen is due to the changes in the conformation of the hemoglobin molecule as it transitions between the deoxygenated and oxygenated states.
Now, let’s address the other options:
- A: When hemoglobin is not bound to oxygen, it does not appear blue. It is typically a darker red or purple color due to the reduced oxygen levels in the blood, not blue.
- B: Hemoglobin is not found in the plasma; it is specifically found in red blood cells. Plasma is the liquid portion of blood that contains water, electrolytes, proteins, and other substances but not hemoglobin.
- C: As mentioned earlier, hemoglobin can bind to a maximum of four oxygen molecules, not three. This is a crucial aspect of its function in oxygen transport.
Thus, option D is the correct statement regarding hemoglobin.